1. Field of the Invention
The present invention relates to an electroluminescence device, which will be referred to as xe2x80x9can organic EL devicexe2x80x9d in this specification hereinafter, used as an element of an organic electroluminescence display. More particularly, the present invention relates to an organic EL device in which resistance of ITO (Indium Tin Oxide) film is reduced by increasing the thickness of ITO film when a transparent conductive film such as ITO film is formed as an anode so that short circuit between the anode and the cathode can be effectively prevented. Also, the present invention relates to a method of manufacturing the organic EL device.
2. Description of the Related Art
Concerning the organic EL device used for a back light of a liquid crystal display, a display applied to various types of display units and a light source of optical communication, by controlling its luminescent material and layer structure, it is possible to obtain various luminescent wave length including blue luminescence, which is difficult to obtain by a conventional inorganic EL device. Therefore, the organic EL device is widely used in the fields of various luminescent devices and color displays.
According to the basic method of manufacturing an organic EL device, it is manufactured as follows. For example, ITO film, which is known as a transparent conductive film, is formed on the surface of a glass substrate so that ITO film can be used as a transparent anode. Organic thin film is laminated on this ITO film. Further, a cathode, which composes a pair together with the anode formed by ITO film, is formed on this organic thin film by means of metallic vapor deposition.
When the size of a display composed of pixels of the above organic EL devices is increased, the following problems may be encountered. The overall ITO film is formed in a wide range. Therefore, resistance of ITO film is increased. As a result, it is impossible to avoid an increase in applied voltage. Also, when it is necessary to provide a higher resolution, the pattern width is reduced, and resistance is increased. Therefore, voltage is increased in the same manner as that described above. For the above reasons, in order to apply the organic EL devices to an image plane of a large size, it is necessary to reduce specific resistance of ITO to be used, or alternatively, it is effective to increase the film thickness of ITO film. This is owing to a reason that the larger the thickness of ITO film is, the lower the resistance is.
However, according to the physical property of ITO film, specific resistance of ITO film can be reduced to a predetermined value (p=150 to 200 xcexcxcexa9xc2x7cm) Therefore, in order to apply the organic EL device to an image plane of a large size, the countermeasure which has been taken up to this time is not sufficient. And also in order to increase the resolution of an image plane, the countermeasure which has been taken up to this time is not sufficient.
On the other hand, by increasing the thickness of ITO film, it is possible to reduce the resistance sufficiently. However, when the thickness of ITO film is increased, the transmittance is deteriorated. Therefore, it is difficult to obtain a sufficiently high intensity of light because of the deterioration of transmittance. Further, the following new problem may be caused. Since the organic thin film formed on ITO film is thinner than ITO film, when the thickness of ITO film is increased, an anode composed of this ITO film is short-circuited with a metallic cathode formed on the organic thin film.
FIG. 3 is a schematic illustration showing an arrangement of a conventional example in which there is a possibility that the anode is electrically communicated (short-circuited) with the cathode when the thickness of ITO film is increased. In the above arrangement, ITO film 52 is formed on a transparent glass substrate 51 by depositing step and patterning step. On this ITO film 52, there is formed an organic thin film 53 which is an organic EL medium. Further, on this organic thin film 53, there is formed a metallic cathode 54 by means of vapor-deposition.
An upper limit of the thickness of the organic thin film 53 is approximately 200 nm at most. When the thickness of the organic thin film 53 is increased to a value higher than that, problems are caused in the characteristics of the organic thin film 53. On the other hand, in order to apply ITO film 52 to an image plane of a large size and high resolution, it is preferable that the thickness of ITO film is not less than 300 nm. For the above reasons, it is difficult for the organic thin film, which is an insulating material (semiconductor), to cover ITO film 52 completely. Especially, in an edge portion of an upper end of ITO film, a defective coverage of the organic thin film 53 tends to occur. In the case of occurrence of a defective coverage of the organic thin film 53, ITO film 52 and the cathode 54 are short-circuited with each other, and the characteristic is deteriorated. That is, when the thickness of ITO film 52 is increased, an increase in the resistance can be suppressed. Therefore, it is possible to apply it to an image plane of a large size. However, due to the above short circuit, a local luminescent trouble is caused, and further a cross talk is caused by a leakage of electrical current.
As described above, the increase in the thickness of ITO film 52 is effective to maintain resistance low, however, a difference in level between ITO film 52 and a surface of the glass substrate 51 is made, which causes a new problem. For the above reasons, instead of the above manufacturing method, there is provided a manufacturing method shown in FIGS. 4A-4D.
Processes according to manufacturing method are shown in FIGS. 4A-4D First, ITO film 52 is deposited on the glass substrate 51 and patterned to anode patterns by photolithography (FIG. 4A). After that, SiO2 film 55 is formed on the anode patterns of ITO film 52 (FIG. 4B). A resist 56 is coated on the SiO2 film 55 and patterned by photolithography (FIG. 4C). Further, when the SiO2 film 55 is etched by using the resist 56 as a mask and then the resist 56 is removed. Therefore SiO2 film 55 is embedded between the anode patterns of ITO film 52 as shown in FIG. 4D. In this way, a difference in level of the surface of the anode patterns of ITO film 52 from that of other area on the glass substrate 51 can be suppressed.
When SiO2 55 is embedded between the anode patterns of ITO film 52, although it is impossible to make the entire surface flat, it is possible to suppress the occurrence of a defective coverage of the organic thin film 54 at the edge portion of the anode patterns of ITO film 52. Due to the foregoing, when the cathode 54 is formed as shown in FIG. 3, it is possible to prevent a short circuit between anode patterns of ITO film 52 and the cathode 54. Therefore, not only it becomes possible to reduce resistance by increasing the thickness of ITO film 52, but also the occurrence of cross talk caused by short circuit after the formation of films can be suppressed.
However, according to the above manufacturing method, the following processes are required. After ITO film 52 is subjected to etching on the glass substrate 51 by using a resist pattern as the mask, SiO2 55 is formed on the overall surface, and then patterning is conducted in accordance with the mask pattern for patterning the ITO film 52. After that, SiO2 55 is subjected to etching, so that SiO2 55 can be embedded only into a clearance (an interval region) between the anode patterns of ITO film 52. That is, it is necessary to conduct patterning not less than twice for the formation anode patterns of ITO film 52 and embedded patterns of SiO2 55. Further, it is necessary to conduct patterning of SiO2 55 in accordance with anode patterns of ITO film 52. For the above reasons, a high accuracy is required for the mask alignment, which greatly affects the yield of products. Even if the above method is adopted, it is impossible to make the overall surface flat when SiO2 55 is formed.
According to the above manufacturing method, SiO2 55 is dependently formed only for the purpose of preventing the short circuit between ITO film 52 and cathode 54, that is, SiO2 55 nevercontributes to the luminescent characteristics such as a luminescent intensity. Although the manufacturing cost is increased only a little by the material expense of SiO2 55, the manufacturing process becomes complicated because of the forming process of SiO2 55 and the matching process of matching the mask, and further the yield of product is deteriorated.
An object of the present invention is to provide an organic EL device characterized in that: the thickness of ITO film is ensured so that an increase in its electric resistance can be suppressed; and a coverage of the organic thin film is excellently maintained in an edge portion so that short circuit between the first electrode and the second electrode can be prevented. Also, it is an object of the present invention to provide a method of manufacturing the organic EL device.
The present invention provides an organic electroluminescence device comprising:
a plurality of first electrodes of transparent conductive film patterns formed on a transparent substrate;
an organic thin film formed on the first electrodes; and
a second electrode formed on the organic thin film, wherein a recess, which is an interval region between the first electrodes, is embedded with a photo resist.
According to the above structure, even if the thickness of the first electrode is increased, it is possible to prevent the occurrence of defective coverage of the organic thin film at the edge portion of the first electrode. Therefore, the occurrence of a short circuit between the first and the second electrode can be positively prevented.
The present invention provides a method of manufacturing an organic electroluminescence device, comprising the steps of:
forming a transparent conductive film on a transparent substrate;
forming an operational film so as to a light shielding layer;
selectively etching the operational film and the transparent conductive film successively to form a plurality of first electrodes covered with the operational film patterns coincide with the first electrodes;
forming a photosensitive film on an entire surface of the substrate on which the first electrodes and the operational film patterns are formed;
exposing the photosensitive film by injecting a light from a side of the operational film patterns and developing it so that the photosensitive film exposed from the light without being shielded by the operational film patterns, is remained;
removing the operational film;
forming an organic thin film on the first electrodes and the photosensitive film remained; and
forming a second electrode on the organic thin film.
According to the above manufacturing method, the operational film through which no light is transmitted is formed corresponding to the first electrode. Namely, only region of the photoresist film, not covered with the first electrode and the operational film, is exposed and developed to be remained. Therefore it is possible to remain a cured negative type resist film so as to be embedded in the interval region between the first electrodes, without conducting a mask alignment.